With the development of optical communication systems, the demand for highly-functional optical modules (optical components) increases. A waveguide type optical device can realize various kinds of lightwave circuits by forming waveguides on a substrate, which is used as an element of the optical module. For higher functionality of the optical module, a hybrid optical module, in which waveguide type optical devices having different functions are integrated, is realized. An example of a specific optical module includes an RZ-DQPSK (Return to Zero Differential Quadrature Phase Shift Keying) module and the like.
The RZ-DQPSK module has the structure, for example, that PLC (Planar Lightwave Circuit) waveguides each forming an optical waveguide on an Si substrate or a quarts substrate by silica-based glass and an LN waveguide forming optical waveguides on an LN (lithium niobate) substrate by using titanium diffusion are butt-jointed so as to be optically coupled, and the LN waveguide is fixed to amount (refer to FIG. 1A and FIG. 1B). In FIG. 1A and FIG. 1B (corresponding to FIG. 6 in Non-Patent Literature 1), the mount achieves a function of a package accommodating the PLC waveguides, the LN waveguide, and a fiber alignment member. Optical fibers are aligned to the fiber alignment member and are butt-jointed thereto to be optically coupled with the PLC waveguide. Connection interfaces between the fiber alignment member and the PLC waveguide and between the PLC waveguide and the LN waveguide are respectively fixed by an adhesive. In addition, the optical fibers are fixed to the mount in a position of penetrating through the mount by soldering or the like. In such a structure, as a temperature in the periphery of the optical module changes, a thermal strain is generated due to a difference in thermal expansion between the respective materials in each of the connection interfaces between the fiber alignment member and the PLC waveguide and between the PLC waveguide and the LN waveguide, which therefore causes the adhesive to be easily separated. Further, a difference in thermal expansion between each of the PLC waveguide and the LN waveguide, and the mount is generated, thus applying tension stress on the optical fiber. An increase in the tensile stress causes breakdown of the optical fiber. Even if a material of the mount (package) is made of stainless, for example, SUS303 to make a difference in thermal expansion coefficient from the LN smaller, a large difference in the thermal expansion coefficient exists between the PLC, the optical fiber or the like, and the package material. Table 1 shows values of the thermal expansion coefficient. For overcoming this problem, a soft adhesive for relaxation of the thermal stress is used in each of the connection interfaces between the fiber alignment member and the PLC waveguide and between the PLC waveguide and the LN waveguide to prevent the separation therein. In addition, as shown in FIG. 1B, there is adopted a method where the optical fiber is buckled to release the thermal stress to be applied on the optical fiber.
TABLE 1Name of Thermal expansion coefficientComponent(×10−6/K)SUS30317.3LN15.4PLC2.5Optical fiber0.75